Drag and Energy Loss

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The Ford Model T had little need for aerodynamic styling when it rolled onto the road in 1908; it never ran faster than 45 miles per hour (72 km/h). Today’s high-performance cars reach 180 miles per hour (290 km/h), although drivers on open highways usually travel closer to 70 to 90 miles per hour (113–145 km/h). A Model T’s blocky frame would not withstand today’s speeds even if it were engineered to travel that fast. Cars today have
designs that reduce wind resistance and drag, both of which cause a vehicle to use more power and more fuel to maintain speed.

Aerodynamic drag consists of forces that work against a vehicle’s forward motion. Examples of things that increase a car’s drag are the following: shape, area, density, speed, incline, viscosity (density of the air), and surface friction. Aerodynamic drag accounts for about 20 percent of the energy consumed by most car models. For this reason engineers study drag in all new vehicle designs to optimize energy efficiency.

All cars, ships, boats, and airplanes have been designed based on a drag coefficient (Cd), which is a unitless value used to describe the aerodynamics of a vehicle in motion. Put another way, Cd describes the relationship between the air’s motion and a vehicle’s motion. Car drag coefficients range from 0.25 to 0.45, with sleek Corvette-style cars on the low end of the Cd range and boxy Hummers at the high end. Usually only the newest prototypes achieve a Cd of 0.25, although the Prius measures 0.26. Car designers and engineers reduce drag and therefore enhance fuel efficiency by the following actions:

1. remove sharp corners from the body design
2. reduce the frontal area
3. remove extra components such as spoilers and roof racks
4. avoid wide tires
5. increase windshield angle
6. remove high axle to lower the car’s body

The best way to reduce drag in a well-designed vehicle is to reduce driving speed. Cars use up most of their energy overcoming the effects of drag that increases disproportionately at higher speeds, meaning a small  increase in speed causes a large increase in drag. Car designers have used race cars as examples of low-drag design. Race car design incorporates
two critical factors for achieving high speeds with a minimum amount of drag: stability and downforce. At speeds of more than 200 miles per hour (322 km/h), race cars must have adequate stability and downforce, which keeps the car from going airborne, a situation called lift. Passenger vehicles travel at speeds that remove the risk of lift, and they remain stable at legal speed limits. Auto industry engineers nevertheless consider stability
and downforce with each new design.